In the control of systems having rotating drive shafts, torque and speed are the fundamental parameters of interest. Therefore, the sensing and measurement of torque in an accurate, reliable, and inexpensive manner has been an objective for decades. With the development of prototype electric power steering systems in which an electric motor driven in response to the operation of a vehicle steering wheel controls the production torque by control of the supply current thereto, the need for a torque sensing apparatus which can accurately detect a torque produced by a steering shaft has been highlighted. Although strides have been made, there remains a need for an inexpensive torque sensing device capable of continuous torque measurements over extended periods of time despite severe environments and operating conditions.
Previously, torque measurement was accomplished by contact-type sensors attached to the shaft. More recently, non-contact torque sensors of the magnetostrictive type have been developed for use with rotating shafts. For example, U.S. Pat. No. 4,896,544 to Garshelis discloses a sensor comprising a torque carrying member, with an appropriately ferromagnetic and magnetostrictive surface, two axially distinct circumferential bands within the member that are endowed with respectively symmetrical, helically directed residual stress induced magnetic anisotropy, and a magnetic discriminator device for detecting, without contacting the torque member, differences in the response of the two bands to equal, axial magnetizing forces. Most typically, magnetization and sensing are accomplished by providing a pair of excitation or magnetizing coils overlying and surrounding the bands, with the coils connected in series and driven by alternating current. Torque is sensed using a pair of oppositely connected sensing coils for measuring a difference signal resulting from the fluxes of the two bands. Unfortunately, providing sufficient space for the requisite excitation and sensing coils on and around the device on which the sensor is used has created practical problems in applications where space is at a premium. Also, such sensors appear to be impracticably expensive for use on highly cost-competitive devices such as in automotive applications.
More recently, torque transducers based on measuring the field arising from the torque induced tilting of initially circumferential remanant magnetizations have been developed which, preferably, use a think wall ring ("collar") serving as the field generating element. See, for example, U.S. Pat. No. 5,351,555 and U.S. Pat. No. 5,520,059 to Garshelis. Tensile "hoop" stress in the ring, associated with the means of its attachment to the shaft carrying the torque being measured establishes a dominant, circumferentially directed uniaxial anisotropy. Upon the application of torsional stress to the shaft, the magnetization reorients and becomes increasingly helical as torsional stress increases. The helical magnetization resulting from torsion has both a circumferential component and an axial component, the magnitude of the axial component depending entirely on the torsion. One or more magnetic field vector sensors, which may comprise flux-gate magnetometers, sense the magnitude and polarity of the field arising, as a result of the applied torque, in the space about the transducer and provides a signal output reflecting the magnitude of the torque.
Flux-gate Magnetometers are known. Such devices measure the strength of external magnetic fields by measuring changes in the inductance of a saturable-core inductor, often referred to as a flux-gate. The flux-gate inductor is driven by an alternating current signal having, for example, a sinusoidal or triangular waveform. The AC input current induces an alternating magnetic field within the flux-gate core. The input signal has sufficient amplitude such that the induced current is large enough to drive the flux-gate core into saturation with each cycle of the input waveform. External magnetic fields are detected by measuring changes to the inductance of the flux-gate coil resulting from an external magnetic field.
When the flux-gate core becomes magnetically saturated, the magnetic permeability of the core drops toward unity, and the inductance of the flux-gate coil drops to a fraction of its original value. The rapid decrease in inductance causes a corresponding drop in voltage across the flux-gate inductor. By monitoring the voltage across the flux-gate inductor, the time when the magnetic flux-density within the flux-gate core reaches saturation can be determined in relation to the alternating cycle of the input waveform.
The magnetic flux density within the flux-gate core is a function of both the induced current flowing through the flux gate inductor and any stray magnetic flux associated with the presence of an external magnetic field. Since the external magnetic field component is variable, the saturation current I.sub.SAT necessary to drive the flux-gate core into saturation depends on the magnitude and direction of the external magnetic field. Also, since the voltage waveform across the flux-gate inductor drops when the flux-gate core reaches saturation, the saturation current, I.sub.SAT, which drives the flux-gate core into saturation can be determined by comparing the output voltage waveform to the input current waveform, and measuring the delay between the rise in the input current waveform and the collapse of the output voltage waveform. Based on these measured changes in the saturation current, the magnitude and direction of the external magnetic field can be derived.
Prior art flux-gate magnetometers are constant amplitude, alternating current devices. In other words, current is flowing through the flux-gate inductor throughout each cycle of the input voltage waveform. As noted, the magnitude of the saturation current is derived by monitoring the timing of the collapse of the voltage waveform across the flux-gate inductor as the flux-gate core reaches saturation. This has typically been accomplished by placing a resistor in series with the flux-gate input, and grounding the flux-gate output. The series resistance is selected to be larger than the reactance of the flux-gate inductor such that when the circuit is fed by a voltage waveform, the current through the circuit is determined mainly by the resistor rather than the inductance of the flux-gate coil. The input to the flux-gate coil is also connected to one input of a voltage comparator, which monitors the voltage across the flux-gate inductor. In this arrangement, alternating current continually flows through the resistor and flux-gate combination, and therefore, power is continually dissipated across the resistor.
The shape of the voltage across the flux-gate resembles the inductor current signal, but is advanced by 90.degree.. In general, the magnetometer circuit is driven by a sinusoid of magnitude sufficient to drive the flux-gate into saturation each half cycle. As noted, when the current through the flux-gate reaches saturation, the inductance of the coil drops such that the voltage across the flux gate drops to 0V while the flux-gate remains saturated. However, since the flux gate is not a perfect inductor, parasitic resistance and inductance within the coil will cause the flux-gate voltage to have a slight slope while the flux-gate core is saturated, and a definite zero crossing can be ascertained. This zero crossing is detected by the comparator connected to the input of the flux-gate. From the timing of the zero crossings relative to the input signal, the magnitude of the saturation current is ascertained. The comparator output is compared to the input voltage to determine the relationship between the zero crossings and the input voltage. Since variations in the external magnetic field alter the saturation current, the drop in the voltage, and thus the zero crossings detected by the comparator, occur at different times relative to the input waveform. By comparing the comparator output signal to the input, the magnitude and direction of the external magnetic field can be derived.
Prior art flux-gate magnetometers provide linear, accurate and noise immune measure of magnetic fields. The operating characteristics of flux-gate magnetometers are favorable when compared to Hall effect and magneto-resistive devices except in their power consumption. Since the magnetometer is a constant current device, power is continually dissipated by the device. The current supplied saturates the flux-gate core; typically this is on the order of tens of milliamps. In applications where the magnetometer is battery powered, current consumption must be kept to a minimum to conserve the battery. Therefore, it is desirable to provide a magnetometer having the positive characteristics of traditional flux-gate magnetometers, including linearity, accuracy and noise immunity, while drawing a negligible current.
One magnetometer made by the assignee of the present invention excites a saturable, flux-gate with a periodic, triangular current waveform, and observes the resultant voltage across the flux-gate. Net flux within the flux-gate results in a second-harmonic of the excitation waveform. A bias-field feedback loop creates a linear and temperature resistant system.
Other designers use the second harmonic to measure magnetic fields. See U.S. Pat. No. 5,696,575 "Digital Flux Gate Magnetometer" and U.S. Pat. No. 5,652,512, "Advanced Digital Flux Gate Magnetometer"--both assigned to Hughes--are two such devices. The Hughes patents are written in the context of a three-winding flux-gate coil. U.S. Pat. No. 5,530,349 by inventors Lopez et al "Magnetic Field Sensor Device Using Flux Gate Principle" shows another device for measuring magnetic field through a second harmonic.
The present invention departs from prior art circuitry in that all signals driving the flux gate coil, and its feedback coil, are derived from digital signals, and digital gates acting as switching power supply components. This is to reduce the power dissipation within the active circuitry over prior art. Unlike the prior art Hughes patents, no digital accumulator or digital-to-analog converter is used.